Spiral housings in various forms are known from the prior art. The published documents DE 8308512 U1, DE 102009050684 A1 or DE 10017808 B4 disclose exemplary designs.
In radial fans, the function of the spiral housings is to collect the air flow at the outlet of the radial fan, allow it to flow in one direction and act as secondary guiding geometry. In the process, the conversion from dynamic pressure back to static pressure occurs. In the boundary area of the main spiral body and in the outlet, a spiral housing has a constriction which is referred to as housing tongue in professional circles. It is known that this housing tongue must be arranged close to the fan impeller in order to achieve a good efficiency of the radial fan. However, due to interaction of the rotating fan blades of the fan impeller with the housing tongue, this leads to strong tonal components in the noise spectrum of the fan (rotation-induced sound), which are perceived as disturbing and which increase the noise level considerably.